Full color thick film dielectric electroluminescent displays, employing thin film phosphors and thick film dielectric layers, provide a greater luminance and superior reliability over traditional thin film electroluminescent displays. However, the phosphor materials, insulator materials and thick film dielectric layer employed in these displays are susceptible to degradation due to reaction with water and other atmospheric vapors. Furthermore, the thick film dielectric layer may act as a reservoir for water and other contaminants that may react adversely with the display structure during its operation. In general, atmospheric contaminants are known to shorten the life of electroluminescent displays and thus in order to protect and minimize damage to these electroluminescent displays various types of seals have been developed for incorporation into displays.
U.S. Pat. No. 6,771,019 (the disclosure of which is incorporated herein in its entirety by reference) discloses the use of perimeter seals in thick film dielectric electroluminescent displays. Briefly, thin film phosphors are typically sandwiched between a pair of addressable electrodes and fabricated on a heat resistant substrate that is also impervious to water and atmospheric contaminants. The phosphor materials are activated by application of an electric field generated between the electrodes. A chemically impervious cover plate is typically placed over the fabricated display and sealed between the substrate and the cover plate with a perimeter seal in order to protect the phosphor material, dielectric layers and electrodes between the substrate and the cover plate. In some cases, the cover plate is on the viewing side of the display, in which case it must be optically transparent, and in other cases, the display is constructed on an optically transparent viewing-side substrate and the cover plate is positioned opposite the viewing side.
The effectiveness of perimeter seals is limited by the tendency of perimeter seals to fail by loss or lack of adhesion between the sealing material and the display substrate and/or cover plate due to stress generated due to differential thermal expansion between the display substrate and the cover plate for the display or by an externally applied mechanical stress.
To minimize ingress of atmospheric contaminants into the display structure a desiccant may be incorporated into the perimeter seal between the display substrate and the cover plate as exemplified by Applicant's co-pending International Patent Application serial number WO2004/067676 (the disclosure of which is incorporated herein in its entirety), however, the desiccant has a finite capacity to absorb these contaminants.
Conformal seal designs have also been developed that employ laminate structures applied to the viewing side of the display consisting of a laminate of one or more bi-layer structures comprising a polymer smoothing and stress relief layer on which is deposited an inorganic film that acts as a diffusion barrier to water and other atmospheric contaminants originating from the ambient environment. However the thickness of the laminated structure is limited by the optical transmissivity of the structure and may not provide a totally impervious barrier to outside contaminant vapour species.
Sealing layers have also been developed for use with other types of displays such as OLEDs as described for example in U.S. Pat. Nos. 5,920,080, 6,146,225, 6,268,695, 6,406,802, 6,891,330 and 6,896,979 and U.S. application serial numbers 2005/0238908, 2005/0248270 and 2005/0276947.
While the aforementioned references may teach the use of various types of seals and seal arrangements for electroluminescent displays, these seals and seal arrangements are not adequate for thick film dielectric electroluminescent displays. Known seals may not adequately immobilize the flux of atmospheric contaminants into the electroluminescent displays over the intended life of the display. They may also not adequately maintain the partial pressure of various vapour species within the display structure to minimize degradation of the display structure due to chemical reactions that produce vaporous reaction products. Often such reactions can be suppressed by maintaining an adequate partial pressure of these vaporous reaction products within the display structure. If the display seal is a perimeter seal, internal pressure within the display structure may increase as the display is stored or operated and may cause a bulging or separation between the display substrate and its cover glass, causing optical distortions in a high resolution colour display due to optical parallax effects and ultimately seal failure. Further, mechanical stress or stress due to temperature variation across a display or rapid temperature change exerted on a display may cause failure of a perimeter seal.
Therefore there still remains a need for an effective seal and sealing process for thick film dielectric electroluminescent displays in order to improve their operating stability and overall reliability overcomes some of the disadvantages of the prior art. The present invention solves a number of problems inherent in sealing technology for thick dielectric displays as taught in the prior art without causing significant impairment of the image quality of the display.